The present invention relates to a speed change control apparatus for belt-driven continuously variable transmissions having a single cylinder as a primary cylinder for driving a primary pulley.
Conventionally, some of the belt-driven continuously variable transmissions (hereinafter, referring CVT) for automotive vehicles are constructed in such a manner that a metallic belt is wound between a primary (driving) pulley and a secondary (driven) pulley. The primary pulley is mounted on a primary shaft on the driving side, and has a pulley groove of a variable width. The secondary pulley is mounted on a secondary shaft on the driven (or following) side, and has a pulley groove of a variable width. Pulley diameters of the primary pulley and the secondary pulley may be varied by hydraulic pressure so as to continuously change the number of rotation of the secondary shaft.
The speed change control of the CVT is carried out by controlling the hydraulic pressure supplied to the primary cylinder provided on the primary pulley and the secondary cylinder provided on the secondary pulley. The hydraulic pressure supplied to the respective cylinders are generated by the oil pump driven by an engine. The line pressure supplied to the secondary cylinder, or the secondary pressure, is adjusted by the line pressure-adjusting valve, and the primary pressure supplied to the primary cylinder is adjusted by the primary pressure-adjusting valve with a line pressure as an original pressure. By adjusting the primary pressure to the value corresponding to the target speed change ratio or the speed-change velocity, the width of the groove of the primary pulley is changed to control the vehicle speed, and the line pressure is adjusted to the value corresponding to the transmissible capacity required to the belt.
Since the CVT of such a hydraulic system is adapted to adjust the primary pressure by depressurizing the line pressure, the primary pressure never exceeds the line pressure. Therefore, the effective area of the primary cylinder is determined to be larger than, about twice that of the secondary cylinder, since the up-shifting will be difficult unless otherwise the effective area of the primary cylinder is larger than the effective area of the secondary cylinder, in order to control the speed change by a hydraulic force of the primary cylinder.
Therefore, when the primary cylinder consists of a single cylinder, the diameter of the primary cylinder have to be larger than that of the secondary cylinder, which results in increase in the capacity of inertia of the primary cylinder.
Attempts have been made in the related art to construct the primary cylinder in a duplicated structure, or in double-cylindered structure, as in Japanese Patent Laid-Open No. 196749/1998. According to this technology, the effective area of the primary cylinder can be secured without increasing the diameter of the cylinder. However, the structure of the primary cylinder becomes complex, which results in increase in cost.
Accordingly, it is the object of the present invention to provide a primary cylinder in a single structure, while maintaining the small diameter of the primary cylinder.
The object can be achieved by a speed change control apparatus for continuously variable transmissions, according to the present invention, having a primary pulley to be mounted on a primary shaft and having a pulley groove with a variable width, a secondary pulley to be mounted on a secondary shaft and having a pulley groove with a variable width, a belt wound on both of that primary pulley and that secondary pulley, a primary cylinder mounted on the primary pulley and provided with a primary hydraulic chamber, and a secondary cylinder mounted on the secondary pulley and provided with a secondary hydraulic chamber. The speed change control apparatus comprises:
a line pressure adjusting valve for adjusting hydraulic fluid supplied from a oil pump into a line pressure;
a primary pressure adjusting valve for adjusting the line pressure into a primary pressure; and
oil passage switching mechanism for switching an oil passage between a first condition and a second condition, wherein in the first condition, the line pressure is supplied to the secondary hydraulic chamber and the primary pressure is supplied to the primary hydraulic chamber, and wherein in the second condition, the line pressure is supplied to the primary hydraulic chamber and the primary pressure to the secondary hydraulic chamber.
In the speed change control apparatus for continuously variable transmissions, according to the present invention, it is advantageous that the effective area of the primary cylinder is determined to be almost the same as the effective area of the secondary cylinder, and also that the oil passage switching means switches the oil passage in the region of speed change ratio in which the belt winding diameters for the primary pulley and for the secondary pulley are almost the same.
Further, in the speed change control apparatus for continuously variable transmissions, according to the present invention, it is advantageous that the effective area of the primary cylinder is determined to be 5 to 60% larger than the effective area of the secondary cylinder.
Furthermore, in the speed change control apparatus for continuously variable transmissions according to the present invention, it is preferable to further comprise a controller for carrying out a speed change control in such a manner that the oil pressure to be supplied to the primary hydraulic chamber is adjusted in the low-speed region and that the oil pressure to be supplied to the secondary hydraulic chamber is adjusted in the high-speed region.